1PAM format specification(5) File Formats Manual PAM format specification(5)
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6 pam - Netpbm common 2-dimensional bitmap format
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10 The PAM image format is a lowest common denominator 2 dimensional map
11 format.
12 It is designed to be used for any of myriad kinds of graphics, but can
14 theoretically be used for any kind of data that is arranged as a two
15 dimensional rectangular array. Actually, from another perspective it
16 can be seen as a format for data arranged as a three dimensional array.
17 The name "PAM" is an acronym derived from "Portable Arbitrary Map."
19 This derivation makes more sense if you consider it in the context of
20 the other Netpbm format names: PBM, PGM, and PPM.
21 This format does not define the meaning of the data at any particular
23 point in the array. It could be red, green, and blue light intensities
24 such that the array represents a visual image, or it could be the same
25 red, green, and blue components plus a transparency component, or it
26 could contain annual rainfalls for places on the surface of the Earth.
27 Any process that uses the PAM format must further define the format to
28 specify the meanings of the data.
29 A PAM image describes a two dimensional grid of tuples. The tuples are
31 arranged in rows and columns. The width of the image is the number of
32 columns. The height of the image is the number of rows. All rows are
33 the same width and all columns are the same height. The tuples may
34 have any degree, but all tuples have the same degree. The degree of
35 the tuples is called the depth of the image. Each member of a tuple is
36 called a sample. A sample is an unsigned integer which represents a
37 locus along a scale which starts at zero and ends at a certain maximum
38 value called the maxval. The maxval is the same for every sample in
39 the image. The two dimensional array of all the Nth samples of each
40 tuple is called the Nth plane or Nth channel of the image.
41 Though the basic format does not assign any meaning to the tuple val‐
43 ues, it does include an optional string that describes that meaning.
44 The contents of this string, called the tuple type, are arbitrary from
45 the point of view of the basic PAM format, but users of the format may
46 assign meaning to it by convention so they can identify their particu‐
47 lar implementations of the PAM format. Some tuple types are defined as
48 official subformats of PAM. See Defined Tuple Types ⟨#tupletype⟩ .
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52 It is easy to get confused about the relationship between the PAM for‐
53 mat and PBM, PGM, PPM, and PNM. Here is a little enlightenment:
54 "PNM" is not really a format. It is a shorthand for the PBM, PGM, and
56 PPM formats collectively. It is also the name of a group of library
57 functions that can each handle all three of those formats.
58 "PAM" is in fact a fourth format. But it is so general that you can
60 represent the same information in a PAM image as you can in a PBM, PGM,
61 or PPM image. And in fact a program that is designed to read PBM, PGM,
62 or PPM and does so with a recent version of the Netpbm library will
63 read an equivalent PAM image just fine and the program will never know
64 the difference.
65 To confuse things more, there is a collection of library routines
67 called the "pam" functions that read and write the PAM format, but also
68 read and write the PBM, PGM, and PPM formats. They do this because the
69 latter formats are much older and more popular, so even a new program
70 must work with them. Having the library handle all the formats makes
71 it convenient to write programs that use the newer PAM format as well.
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75 A convenient way to read and write the PAM format accurately is via the
76 libnetpbm(1) C subroutine library.
77 A PAM file consists of a sequence of one or more PAM images. There are
79 no data, delimiters, or padding before, after, or between images.
80 Each PAM image consists of a header followed immediately by a raster.
82 Here is an example header:
84 <span style="font-family: monospace">
86 P7
87 WIDTH 227
88 HEIGHT 149
89 DEPTH 3
90 MAXVAL 255
91 TUPLTYPE RGB
92 ENDHDR
93 </span>
94 The header begins with the ASCII characters "P7" followed by newline.
96 This is the magic number.
97 Note: xv thumbnail images also start with the "P7" magic number. (This
99 and PAM were independent extensions to the Netpbm formats). The rest
100 of the format makes it easy to distinguish PAM from that format,
101 though).
102 The header continues with an arbitrary number of lines of ASCII text.
104 Each line ends with and is delimited by a newline character.
105 Each header line consists of zero or more whitespace-delimited tokens
107 or begins with "#". If it begins with "#" it is a comment and the rest
108 of this specification does not apply to it.
109 A header line which has zero tokens is valid but has no meaning.
111 The type of header line is identified by its first token, which is 8
113 characters or less:
114 ENDHDR This is the last line in the header. The header must contain
118 exactly one of these header lines.
119 HEIGHT The second token is a decimal number representing the height of
122 the image (number of rows). The header must contain exactly one
123 of these header lines.
124 WIDTH The second token is a decimal number representing the width of
127 the image (number of columns). The header must contain exactly
128 one of these header lines.
129 DEPTH The second token is a decimal number representing the depth of
132 the image (number of planes or channels). The header must con‐
133 tain exactly one of these header lines.
134 MAXVAL The second token is a decimal number representing the maxval of
137 the image. The header must contain exactly one of these header
138 lines.
139 TUPLTYPE
142 The header may contain any number of these header lines, includ‐
143 ing zero. The rest of the line is part of the tuple type. The
144 rest of the line is not tokenized, but the tuple type does not
145 include any white space immediately following TUPLTYPE or at
146 the very end of the line. It does not include a newline. There
147 must be something other than white space after the TUPLTYPE
148 token.
149 If there are multiple TUPLTYPE header lines, the tuple type is
151 the concatenation of the values from each of them, separated by
152 a single blank, in the order in which they appear in the header.
153 If there are no TUPLTYPE header lines the tuple type is the null
154 string.
155 The raster consists of each row of the image, in order from top to bot‐
159 tom, consecutive with no delimiter of any kind between, before, or
160 after, rows.
161 Each row consists of every tuple in the row, in order from left to
163 right, consecutive with no delimiter of any kind between, before, or
164 after, tuples.
165 Each tuple consists of every sample in the tuple, in order, consecutive
167 with no delimiter of any kind between, before, or after, samples.
168 Each sample consists of an unsigned integer in pure binary format, with
170 the most significant byte first. The number of bytes is the minimum
171 number of bytes required to represent the maxval of the image.
172 The character referred to as "newline" herein is the character known in
174 ASCII as Line Feed or LF.
175
178 Height, width, depth, and maxval are at least 1.
179 Height, width, and depth have no defined maximum, but processors and
181 generators of images usually have their own limitations.
182 The maxval of an image is never greater than 65535. (The reason it is
184 limited is to make it easier to build an image processor, in which
185 intermediate arithmetic values often have to fit within 31 or 32 bits).
186 There was no specified limitation before October, 2005, but essentially
187 all implementations have always observed it.
188
191 Some tuple types are defined in this specification to specify official
192 subformats of PAM for especially popular applications of the format.
193 Users of the format may also define their own tuple types, and thus
194 their own subformats.
195 Tuple type affects only the meanings of the samples (which are unsigned
197 integers) in the tuples of the image. It does not affect how the sam‐
198 ples or tuples are encoded. Tuple type may affect the meaning of a
199 tuple's position in the array (e.g. it may indicate in a visual image
200 that a tuple in Row 1 is one at the top of the image rather than the
201 bottom).
202 Tuple type never determines how many samples are in a tuple (that is
204 instead determined by the DEPTH header line). Tuple type could be said
205 to imply a depth (number of samples per tuple) because certain tuple
206 types are valid only in combination with certain DEPTH values, but it
207 is good programming practice to use DEPTH for the depth when decoding
208 the raster and separately validate that the depth is consistent with
209 the tuple type. Also, it is good practice to accept a depth that is
210 too great and just ignore the higher numbered planes.
211 PAM Used For Visual Images
214 A common use of PAM images is to represent visual images such as are
215 typically represented by images in the older and more concrete PBM,
216 PGM, and PPM formats.
217 Black And White
219 A black and white image, such as would alternatively be represented by
221 a PBM image, has a tuple type of "BLACKANDWHITE". Such a PAM image has
222 a depth of 1 and maxval 1 where the one sample in each tuple is 0 to
223 represent a black pixel and 1 to represent a white one. The maxval,
224 height, width, and order of tuples in the raster bear the obvious rela‐
225 tionship to those of the equivalent PGM image.
226 Note that in the PBM format, a sample value of zero means white, but in
228 PAM, zero means black.
229 Grayscale
231 A grayscale image, such as would alternatively be represented by a PGM
233 image, has a tuple type of "GRAYSCALE". Such a PAM image has a depth
234 of 1. The maxval, height, width, and raster bear the obvious relation‐
235 ship to those of the equivalent PGM image.
236 Color
238 A color image, such as would alternatively be represented by a PPM
240 image, has a tuple type of "RGB". Such a PAM image has a depth of 3.
241 The maxval, height, width, and raster bear the obvious relationship to
242 those of the PPM image. The first plane represents red, the second
243 green, and the third blue.
244 Transparent
246 Each of the visual image formats mentioned above has a variation that
248 contains transparency information. In that variation, the tuple type
249 has "_ALPHA" added to it (e.g. "RGB_ALPHA") and one more plane. The
250 highest numbered plane is the opacity plane (sometimes called an alpha
251 plane or transparency plane).
252 In this kind of image, the color represented by a pixel is actually a
254 combination of an explicitly specified foreground color and a back‐
255 ground color to be identified later.
256 The planes other than the opacity plane describe the foreground color.
258 A sample in the opacity plane tells how opaque the pixel is, by telling
259 what fraction of the pixel's light comes from the foreground color.
260 The rest of the pixel's light comes from the (unspecified) background
261 color.
262 For example, in a GRAYSCALE_ALPHA image, assume Plane 0 indicates a
264 gray tone 60% of white and Plane 1 indicates opacity 25%. The fore‐
265 ground color is the 60% gray, and 25% of that contributes to the ulti‐
266 mate color of the pixel. The other 75% comes from some background
267 color. So let's assume further that the background color of the pixel
268 is full white. Then the color of the pixel is 90% of white: 25% of
269 the foreground 60%, plus 75% of the background 100%.
270 The sample value is the opacity fraction just described, as a fraction
272 of the maxval. Note that it is not gamma-adjusted like the foreground
273 color samples.
274
278 No Internet Media Type (aka MIME type, content type) for PBM has been
279 registered with IANA, but the unofficial value <span style="font-fam‐
280 ily: monospace">image/x-portable-arbitrarymap</span> is assigned by
281 this specification, to be consistent with conventional values for the
282 older Netpbm formats.
283
286 The conventional suffix for the name of a PAM file is ".pam". But this
287 is not required.
288
292 Netpbm(1), pbm(1), pgm(1), ppm(1), pnm(1), libnetpbm(1)
293
295 This manual page was generated by the Netpbm tool 'makeman' from HTML
296 source. The master documentation is at
297 http://netpbm.sourceforge.net/doc/pam.html
299netpbm documentation 27 November 2013 PAM format specification(5)